Presence of bodies or objects may be detected by determining a change of capacitance between two plates. The presence of an object causes a change in the dielectric constant between the plates, which in turn causes a change in the capacitance formed by said two plates, when compared with a situation where the object is far away from said plates.
A capacitive sensor may be used e.g. to detect movements of people e.g. in an anti-theft alarm system.
The absolute value of the capacitance of a capacitive sensor is typically very small. Electro-magnetic noise coupled into the sensor and to a monitoring circuit makes it difficult to detect small changes in said capacitance.
It is known that the capacitance value of a capacitor may be measured by coupling said capacitor as a part of an RC-circuit, and by determining the time constant of said RC-circuit. The resistor and the capacitor are connected in series, and the capacitor is charged through the resistor, starting from a defined voltage. The charging time can be characterized with the time constant. The time constant of the circuit, formed by the capacitor and the resistor, is determined either by measuring the time until a predetermined voltage level is reached or by measuring the voltage after a predetermined loading time. When the time constant and the resistance are known, the capacitance can be calculated.
This method can be used in measuring the capacitance of a capacitive sensor. A problem of the method is that the energy of a measured signal is very low, if the measured capacitance is low. Therefore, it is difficult to attain sufficient precision by measuring the charging time or the voltage attained after a predetermined loading time. Furthermore, electromagnetic radiation can easily interfere with the measurement. In practice, the capacitance of the sensor is so low that the charging time is also short and cannot be measured accurately enough e.g. by using a low-cost micro controller. Furthermore, a measurement based on this principle does not contain any kind of low-pass filter, which allows aliased high-frequency noise to appear on top of the signal to be measured.
It is known that the capacitance value of a capacitor may be measured by coupling an alternating voltage to said capacitor, and by determining the impedance of said capacitor.
The capacitor resists alternating current flow due to its impedance. The impedance is inversely proportional to the capacitance in a frequency domain. The impedance of the unknown capacitor can be compared with the impedance of a known capacitor by using, for example, a bridge comparison circuit, such as the Wheatstone bridge. This method requires complicated circuits and is therefore expensive.
It is known that changes in the capacitance value of a capacitor may be detected by coupling said capacitor as a part of a tuned oscillation circuit.
A capacitive sensor arrangement may comprise a resonance circuit composed of an unknown sensor capacitor and a known coil (inductance). When the capacitance of the sensor capacitor reaches a defined value, the circuit starts to resonate and the amplitude of the oscillation increases suddenly. It can be easily measured whether the circuit is resonating or not. This method is extremely sensitive, but only in a certain narrow capacitance range. When a wider range is required, this method is not practicable.